The present invention relates generally to a drive roller unit for driving articles to be conveyed on a conveyor track.
Roller-type or ball-type conveyor tracks in which articles to be conveyed thereon are supported movably on a plurality of support rollers or balls often include drive roller units for driving the articles to be conveyed, for example frieght containers. A preferred area of use of such arrangements are frieght loading systems in air freight traffic, in which the freight containers and transported in the freight compartment of an aircraft on such conveyor tracks until the containers are at the anchorage locations at which they are to be held fast during a flight.
A drive roller unit as is disclosed for example in U.S. Pat. No. 3,698,539 comprises a drive roller which is driven in rotation by an electric motor by way of a transmission arrangement. The drive roller is mounted rotatably on a guide structure, by means of which it is movable relative to a base construction between a lowered rest position and a raised drive position in which it engages the underside of an article to be driven thereby. The transmission arrangement includes a drive gear which is driven by the electric motor and which meshes with a tooth configuration non-rotatably disposed on the drive roller. The axis of rotation of the drive gear is stationary relative to the base structure. More specifically that unit has a main frame which is to be secured to the floor for example of a freight compartment, and a mounting structure which is mounted at one end pivotably on the main frame and which carries the motor in the vicinity of the axis of pivotable movement. Disposed at a spacing from the axis of pivotal movement is the drive roller which is thus mounted rotatably, being in a stationary position relative to the mounting structure, also including a cam control arrangement which is connected to the free end of the mounting structure and by means of which the mounting structure is pivotable relative to the main frame from the rest position of the drive roller into the operative position. The output shaft of the motor is mounted to the input of a planetary-type transmission having one output which is coupled by way of a transmission to the drive shaft of the drive roller while the other output drives the cam control arrangement.
In the rest condition of the unit the mounting structure lies on the main frame so that the drive roller is in a lowered position at a spacing from the bottom of a freight container which is moved thereover on a roller conveyor track. In order to set the freight container moving on the roller conveyor track, the motor is first switched on, to drive the input of the planetary transmission. At that time the drive roller is prevented from rotating by virtue of the inclusion in the arrangement of a slipping clutch which is set to provide a predetermined level of braking torque so that the drive moment of the motor positively acts by way of the other output of the planetary transmission to drive the cam control arrangement which accordingly, by way of its cams which bear against the main frame, pivots the mounting structure upwardly into the operative position in which the drive roller comes into engagement with the underside of the floor of a feight container. Due to the weight of the freight container, that engagement between the drive roller and the freight container causes the movement of the cam control arrangement to be blocked and the drive roller is pressed against the bottom of the container. However that also causes the output of the planetary transmission which drives the cam control arrangement to be blocked so that the other output of the planetary transmission, which drives the drive roller, now transmits to the drive roller a torque which overcomes the braking torque of the slipping clutch so that the drive roller, in the condition of being pressed against the bottom of the freight container, begins to rotate and the container is thus displaced on the conveyor track in a direction which depends on the direction of rotation of the drive motor.
A problem which arises with that arrangement in a practical situation is that the complete transmission arrangement for transmitting torque from the motor to the drive roller on the one hand and to the cam control arrangement on the other hand is of a very complicated construction and involves a large number of components. In addition, a cam control structure is required so that the overall construction includes a large number of mechanically moving components. That not only gives rise to a high level of manufacturing costs, but it also involves a large structure and also a high weight which is extremely undesirable in the context of an aircraft structure. Furthermore the large number of components which are movable relative to each other gives rise to an undesirably high level of susceptibility to wear and the periods of time between successive maintenance operations are short in order to ensure that the arrangement affords the high level of reliability required in aircraft.
In addition, that drive roller unit uses a slipping clutch in the form of what is referred to as a fluid friction clutch, the operating characteristics of which are highly dependent on temperature. Therefore, due to the high temperature differences which occur in an aircraft freight compartment, it is necessary for the maximum torque of the slipping clutch to be selected at such a high level that the clutch torque does not fall below the minimum value required, even under the most disadvantageous temperature conditions. However that gives rise to very high force peaks when initiating the rotary movement of the drive roller in the condition of being pressed against the bottom of a freight container, and such forces act on the floor structure of the aircraft freight compartment. That is extremely undesirable in a practical situation as specific limit loadings in respect of the floor structure are not to be exceeded and in addition shock loadings are highly undesirable.
Yet another difficulty with the drive roller unit set forth above is that it is of a really large size and the individual components lie more or less unprotected side by side so that in operation the unit is not only liable to suffer from fouling and defects and interruptions in operation as a result thereof, but it is also liable to suffer from damage due to objects which penetrate into the mechanism or by virtue of being directly subjected to an outside force which interferes with the mechanism.
A similar form of drive roller unit is to be found in EP-B1-0 149 658, which also suffers from problems due to foreign bodies penetrating into the mechanism or a force acting thereon to cause it to become defective, besides being of large size.